A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in Equation (1) below:
                              C          ⁢                                          ⁢          D                =                              k            1                    *                      λ                          N              ⁢                                                          ⁢              A                                                          (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from Equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector apparatus (also referred to, hereinafter, as a source collector module or source module) for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector apparatus may include an enclosing structure arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
In addition to radiation, the plasma of a plasma radiation source produces contamination in the form of particles, such as thermalized atoms, ions, nanoclusters, molecules consisting of fuel atoms bonded to buffer gas atoms, and/or microparticles. Such contamination is also referred to as debris, hereinafter. The contamination is output, together with the desired radiation, from the radiation source towards the radiation collector and may cause damage to the normal incidence radiation collector and/or other parts. For example, LPP sources that use tin (Sn) droplets to produce the desired EUV may generate a large amount of tin debris in the form of: atoms, ions, nanoclusters, and/or microparticles.
It is desirable to prevent the contamination from reaching the radiation collector, where it may reduce EUV power, or from reaching parts of the enclosing structure where it may create other problems. To stop especially the ions, a buffer gas can be used, but with this kind of debris mitigation, a large flow of buffer gas may be needed, which may make it desirable to have large pumps and a large supply of buffer gas. To reduce a volume of the desired supply of buffer gas, the enclosing structure of the source collector module may define a closed loop flow path of the buffer gas disposed in the enclosing structure and a pump forcing the gas through the closed loop flow path. A heat exchanger may be used to remove heat from gas flowing in the flow path, and a filter may be used to remove at least a portion of contamination from gas flowing in the flow path.